Stylish was a formerly open-source browser extension that could apply a user-supplied style sheet to a web page, in addition to the Cascading Style Sheets provided by the website itself, to customize and personalize the appearance of the page. A user style may be more or less selective, targeting one specific web page, or several, or all of the pages on one or more domains, or every page on the web.

In July 2018, it was discovered that the extension was tracking its users' browsing history and sending them to a remote server[8][9] operated by its parent company, SimilarWeb. It was subsequently removed from the official Firefox, Chrome, Opera and Safari extension websites and is no longer available to install.

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User styles are CSS code designed to alter the appearance of one, some, or all sites. Stylish for Firefox can additionally style the skin of the browser itself, but the Android version does not support this because the user interface is built in native Android code. The styles are applied only to the targets specified. Individual user styles can be enabled or disabled without having to restart the browser.

User styles are added to the CSS rules provided by the site, but can also override the site's styling (often requiring the !important keyword for each replacement rule). The most common uses are ad-blocking, applying a new color scheme, and eliminating unwanted page elements.[11]

There are three classes of user styles. Site styles change the appearance of a particular web site. Global styles change the appearance of all web sites. App styles change the appearance of the Firefox user interface, only supported on Firefox. It is similar to the userChrome.css CSS file used by Firefox and Mozilla-based browsers.

Since January 2017, following the sale of Stylish to Israeli company SimilarWeb, the extension has contained tracking functions that sends information on all visited URLs and HTTP requests, and information contained on search engine results pages, to SimilarWeb servers. In July 2018, after these issues were publicized by a software engineer, Stylish was pulled from both the Chrome Web Store and Mozilla Add-ons, as well as being automatically uninstalled for all existing users. [12][13][8]

Stylish is often compared to Greasemonkey, another Mozilla extension that allows client-side manipulation of web pages.
Greasemonkey's user scripts are essentially dynamically inserted JavaScript that can alter a page's appearance or functionality, while Stylish uses CSS, which can only alter appearance.

Userstyles.org holds user-generated styles, most of which can be converted to Greasemonkey scripts. These also have the option of being added to Greasemonkey.

Stylus is a compatible browser extension which offers similar features (and more) and does not track any user data.